After doing what I had planned, here is the result, for the benefit of other:

(1) Do I have to suspend the activity at both the Master sites ?
-- I only did a suspend on the Mater Defenition site for that Master Group; that Quiescied the replication process on both the Master sites

(2) Will the DDL get replicated ?
-- Yes, the DDL (alter table add) did get replicated to the other Master site